An antenna links a receiver to its electromagnetic environment. A desired signal incident upon the antenna is processed by the receiver, thereby extracting from it certain information. However, in the presence of sufficiently strong incidental or intentional interference, the desired signal becomes so overwhelmed, that the receiver can no longer properly perform its function. This undesirable situation may be alleviated to a large extent with the use of adaptive array processing.
In adaptive array processing the desired signal is enhanced over an interference by introducing an auxiliary antenna which, upon appropriately weighting (with amplitude and phase) the signal therefrom, will combine with the main antenna signal to form a null in the direction of the interference. This nulling operation is generally done automatically with the use of a feedback controller. An appropriate function of the feedback controller is the minimization of average combined power with respect to the auxiliary weight. In fact, since the nulled auxiliary antenna observes substantially sidelobe power, its influence is confined essentially to the sidelobe power, having minimal effect on the main beam and, thereby, leaving the desired signal practically unperturbed.
Conventional gradient control is described in detail in an article by B. Widrow, et al, "Adaptive Antenna Systems", procedures of the IEEE, Volume 55, Number 12, December 1967. Conventional gradient control is suboptimal in the sense that it incorporates a constant gain instead of optimal gain and as a consequence can guarantee stability only with a sufficiently small gain to the expense of relatively slow convergence. Generally, constant gain gradient control cannot account for the underlying geometry of the power hyperparaboloid.